1. Field of the Invention
The present invention relates to biological treatment of contaminated liquids and effluent, and more particularly to apparatus and methods for efficiently processing waste using an internal-recycle configuration.
2. Description of the Related Art
Biological processes to treat contaminated water take many forms. Generally these involve exposure of the waste stream to one or more forms of microorganism that stabilize or digest various of the contaminants. The microorganisms are chosen to complement the waste stream both in terms of sewage contents and chemical environment, since any species of microorganism favors a particular environment with limited tolerance for variation. For example, the activated sludge process utilizes aerobic bacteria that remove the soluble biological oxygen demand (BOD) from wastewater. Practice of this process generally involves conducting wastewater into an aeration basin containing a suspension of digestive microorganisms, thereby forming a "mixed liquor" that is aerated to furnish oxygen for respiration of biomass; the biomass sorbs, assimilates and metabolizes the BOD of the wastewater. After a suitable period of aeration, the mixed liquor is introduced into a clarifier in which the biomass settles, allowing the treated wastewater to overflow into an outlet effluent stream.
An important aspect of traditional wastewater treatment is adequate agitation of the mixed liquor in order to speed contact between the digestive microorganisms and waste materials, which may be suspended or dissolved in the wastewater. Indeed, an optimal amount of turbulence is generally dictated more by economics than by process requirements; high agitation rates are theoretically the most desirable, but are also expensive to attain. See, e.g., U.S. Pat. Nos. 4,961,854, 4,056,465 and 3,964,998.
An exception to this practice involves the use of fixed-growth media, where the biological organisms are maintained on fixed supports rather than dispersed in suspension. In this case mixing is avoided to prevent shear that might remove the biological attached growth. The application of fixed-growth systems is ordinarily restricted to soluble, non-particulate contaminants; in addition, these processes are limited in loading capacity by the surface area of the biological support and the diffusion characteristics of the waste stream.
Fluidized-bed systems represent a combination of suspension and fixed-growth processes, but require added media for surface area, mixing sufficient to maintain homogeneity of the media and its attached biological growth, and periodic or continuous removal of the media for regeneration.
All of these systems ordinarily are limited to one category of microorganism, since differing biological processes vary significantly in terms of multiplication rates, optimum conditions, and preferred inputs and waste products. Most generally, microorganisms for wastewater treatment include aerobic, anaerobic and anoxic species, all of which are sustained by very different (and mutually inconsistent) environments. Process conditions can also restrict the applicability of a particular biological approach. For example, the optimal biological process for a particular wastewater composition might require a longer solids retention time than that afforded by economically feasible complete-mix processes, and exhibit greater throughput needs than can be met with fixed-film and fluid-bed film reactors.
This is unfortunate, since frequently a combination of biological processes would be ideal for treatment of a particular waste composition. Thus, it might be advantageous to combine both nitrifying and denitrifying agents, but the former require substantial dissolved oxygen while the latter can only tolerate minimal (if any) dissolved oxygen. Although some progress in combining processes has been achieved using facultative lagoons, these constructions generally require acres of surface area, are used to process only small amounts of waste, and remain at the mercy of natural weather conditions that can uncontrollably alter process conditions and affect biological viability.